1. Field of the Invention
The present invention relates to a method of repairing a damaged portion of an optical fiber embedded in a structure made of a composite material, a repairing structure of the embedded optical fiber and a connecting structure of the embedded optical fiber.
2. Description of the Related Art
In recent years, composite material structures have been used in various fields. Since composite materials are smaller in weight and higher in strength and stiffness than metal materials, structures incorporating the composite materials can be reduced in weight. Therefore, for example, the use of the composite materials in structures of aircraft and the like has been expanding.
There may be a chance that an internal damage is generated in the composite material structure due to, for example, a collision of foreign objects against the composite material structure, or the like. Therefore, it becomes necessary to perform a design (dame tolerance design) which allows a damage which is unable to be found out in an inspection or a design (fail safe design) which prevents a damage from becoming fatal before a next inspection is conducted. A damage which is not allowable in design must be found out in inspection and repaired. Therefore, a visual inspection or a non-destructive inspection must be conducted from an inner surface side of fuselage as well as an outer surface of the fuselage. For some composite material structures having a complex shape or a complex curved surface, non-destructive inspection cannot be conducted. Accordingly, as a method of inspecting and diagnosing structural health, for example, the presence/absence of the damage in the composite material structure, for contributing to reduction of structure weight generated in the damage tolerance design and the fail safe design, reduction of cost/reduction of period/prevention of error/easiness of the visual inspection or the non-destructive inspection, there is proposed a damage detection system for detecting the damage or the like by using an optical fiber sensor embedded in the composite material.
As this type of prior art, for example, there is a dame detection device (in the embodiment, including a damage detection device which performs comparison of FBG (Fiber Bragg Grating) sensor measurement values of strains occurred by impact applied to the composite material and analysis values and uses a difference of strain response arrival time), in which a plurality of sensors (strain measurement FBG sensors, etc.) for reflecting optical signals with different frequencies are provided to be spaced apart from each other, in an optical path structure constructed using an optical fiber, such as a composite material structure embedded with the optical fiber, and the presence/absence, level or location of the damage are determined based on the presence/absence and intensities of the optical signals reflected in the sensors (optical intensity measurement), etc. (e.g., see Patent Literature 1: Japanese Laid-Open Patent Application Publication No. 2005-321223).
In the case of using the damage detection method as described above, if a damage occurs in the optical fiber embedded in the composite material, the damaged portion must be repaired. For example, as a repair method of an optical fiber as a single member, there is a method in which the damaged portion is cut, a replacement optical fiber is placed in a portion where the damaged portion is cut, and the replacement optical fiber is connected to the optical fiber by fusion splicing or in a state in which the end surface of the optical fiber and the end surface of the replacement optical fiber are butted with each other. However, the above repair method is unable to repair the optical fiber embedded in composite material structures.
Accordingly, as a repair method in the case where a damage occurs in the optical fiber embedded in the composite material, there is a method shown in FIGS. 14A to 14C. This method is as follows. If a damage 200 occurs in an embedded optical fiber 110 embedded in a composite material 102 of a composite material structure 101 (FIG. 14A), a replacement optical fiber 120 is bonded to the surface of a damaged portion of the composite material 102 by adhesive 121 (FIGS. 14B, 14C), to connect the replacement optical fiber 120 to the embedded optical fiber 110 in a position apart from the damaged position.
As a repairing method in the case where a damage occurs in the optical fiber embedded in the composite material structure, there is a method in which the surface layer of the composite material is scraped out and removed, and the end portion of another optical fiber is joined to the end portion of the optical fiber, which corresponds to the removed portion of the composite material, by fusion-splicing etc. (see e.g., Patent Literature 2: Translated PCT Application Publication No. 2009-517680).
However, in the repairing method shown in FIG. 14, the embedded optical fiber itself is not repaired, and other healthy portion which cannot attain necessary transmitted light due to the damaged portion, as well as the damaged portion, cannot be used and is left. The optical fiber for use as the above stated optical fiber sensor has a core diameter of about 5 μm to 10 μm, and a cladding diameter of about 40 μm to 130 μm, for example. In the configuration in which the replacement optical fiber is bonded to the surface of the composite material and the end portion of the replacement optical fiber is connected to the optical fiber embedded in the composite material, problems associated with durability and reliability of the optical fiber bonded to the surface may sometimes arise. Especially, in the case of a smaller diameter optical fiber, a severe problem associated with durability and reliability might arise. Furthermore, in a certain composite material structure, the optical fiber cannot be placed in the surface. In that case, the above stated method cannot be applied.
In addition, as compared to the case where an optical fiber is embedded, in some cases, it becomes difficult to detect a damage accurately based on the presence/absence, intensity, or the like of an optical signal reflected by the optical fiber sensor.
In the repairing method disclosed in Patent Literature 2, the above stated embedded optical fiber having the damaged portion might be further damaged when the corresponding portion of the composite material is scraped out and removed from the surface layer. Thus, it is difficult to draw out the embedded optical fiber in a healthy state. Because of this, it is difficult to repair the optical fiber sensor to a state in which it is capable of detecting a damage and others accurately again or to repair the optical fiber to a state in which it is capable of attaining necessary transmitted light again.